When a fluted drill bit is used to drill a hole into a work piece, particularly a metal work piece, there is a tendency to create long drill chips. The long drill chips travel up the flutes of the drill bit, particularly if the work piece is metal. Metal chips that travel up the flutes of a drill bit can become tangled around the drill bit or wedged between the drill bit and surrounding hardware. In contrast to long chips, small, short drill chips tend to be forced away from the a drill bit by centrifugal force.
In some environments, such as the fabrication of aircraft, where many holes are drilled into thick aluminum sections, vacuum lines are used to remove metal chips from the area around holes as the holes are being drilled. Long chips can plug the vacuum line. In order to prevent this from occurring, vacuum lines are constantly cleaned with hazardous solvents that may not be available in the future.
In order to overcome the disadvantages associated with long drill chips, various proposals have been made to reduce chip size to make chips more manageable. Past attempts have included varying drill bit speed and drill bit feed rate to create a chip size that breaks up naturally. Unfortunately, this approach results in increased cycle time per hole, a reduction in hole quality and excessive wear on drill bits. As a result, a number of bushing designs and special drill geometries whose purpose is to reduce the size of drill chips have been proposed. None has been entirely successful. One such drill bushing design is described in U.S. Pat. No. 5,108,241, entitled "Chip Breaker Drill Bushing Assembly" by David A. Cross, assigned to the assignee of the present application.
Prior art bushing designs developed to solve the long chip problem by breaking up long chips into smaller chips have created a number of new problems. These include chip packing in the drill template if a template is being used, chips jamming between the drill bit and the drill bit bushing, which prevents the release of the nose piece from the end effector of a robotic arm used to position and operate the drill bit, and increasing the load on the drill motor and its sensors, which affects the performance (quality or cost) of the drilling process and/or monitoring.
The present invention is directed to providing a chip breaker drill bushing assembly that overcomes the foregoing disadvantages of prior art methods and apparatus directed to eliminating the problems associated with long drill chips.